Maleimide-modified high heat ABS resins

ABSTRACT

Graft copolymerized maleimide-modified, impact resistant monovinylidene aromatic copolymer compositions have substantially improved impact strength and fatigue resistance properties when the swelling index thereof is 12 or greater and when the numerical difference in maleimide monomer content as between the grafted copolymer and matrix copolymer portions thereof differ by no more than 9 percentage points from each other. Such compositions are conveniently prepared by mass, solution or mass/suspension graft copolymerization processes and by deferring the addition of at least about 30 percent of the maleimide monomer ingredient to a point in the process after (and preferably only shortly after) phase inversion of the dissolved impact modifying rubbery polymer ingredient.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 08/127,726, filed Sep. 27,1993, now U.S. Pat. No. 5,412,036.

BACKGROUND OF THE INVENTION

This invention pertains generally to impact resistant monovinylidenearomatic copolymer compositions such as those that are commonly known inthe art as ABS resins. In particular, said invention pertains torubber-modified monovinylidene copolymers which can, for example, beprepared by mass, solution or mass/suspension graft copolymerizationtechniques and which have incorporated (i.e., copolymerized) therein oneor more N-substituted maleimide monomers for the purpose of enhancingthe heat resistance (e.g., the softening point, heat distortiontemperature, etc.) of the resulting graft copolymer product.

Maleimide-modified, impact resistant styrenic copolymer compositions arealready known in the art as a general proposition. For example, in U.S.Pat. No. 3,652,726 there are disclosed certain graft copolymerscomprising a diene rubber substrate and a superstrate resin comprisingacrylonitrile, N-aryl maleimide and an aromatic olefin, such as styrene.Also disclosed are blends of such graft copolymer with variouscompatible matrix resins. The reference further teaches at col. 6, line70, that the graft copolymers may be made by a sequential polymerizationin which the monomers for the superstrate are polymerized by a freeradical process. Bulk, suspension, solution, or emulsion polymerizationsare all disclosed as suitable for preparing such polymers. Emulsiontechniques are particularly exemplified.

More recently (i.e., in U.S. Pat. No. 4,808,661) there have beendisclosed maleimide-modified ABS-type compositions that are prepared bycontinuous bulk (or mass) polymerization techniques and which arerequired to meet certain specified criterion and compositionalcharacteristics in order to provide the balance of performanceproperties which are contemplated for the specific purposes of thatpatent. In particular, such patent includes the requirements that:

1. the occluded and/or grafted styrene/acrylonitrile/maleimide (S/AN/MI)polymer contained in or on the dispersed rubbery polymer be in the rangeof 50 to 100 parts per 100 parts of the rubbery polymer;

2. the amount of maleimide in both the grafted/occluded polymer (x) andthe continuous matrix phase (y) be in the 1 to 25 weight percent range;

3. the ratio of y:x be greater than 0.5 and less than 2.0; and

4. the crosslinking degree index (hereinafter also referred to asswelling index) thereof be in the range of from 4 to 11.

SUMMARY OF THE INVENTION

It has now been discovered that substantially improved properties(particularly impact strength and fatigue resistance) are obtained inmaleimide-modified, impact resistant monovinylidene aromatic copolymercompositions of the sort described above when said compositions have orare caused to have swelling index values of 12 or greater. Such findingis considered to be especially surprising in view of the contraryteachings of U.S. Pat. No. 4,808,661 (i.e., at Col. 7, lines 41-43thereof) to the effect that the impact strength of such compositions isreduced when the swelling index thereof exceeds 11.

It has additionally been discovered that the impact strength of thesubject maleimide-modified copolymer compositions is also notablyimproved by ensuring that the maleimide monomer content as between thecontinuous matrix phase ("Y" in weight percent on a matrix phase weightbasis) and the grafted/occluded copolymer constituent ("X" in weightpercent on a grafted/occluded copolymer weight basis) of such copolymercompositions is sufficiently balanced such that the numerical differencetherein (i.e., the absolute value of X-Y) is no more than 9 weightpercentage points.

In light of the foregoing discoveries, the present invention in one ofits aspects is a rubber-modified monovinylidene aromatic copolymercomposition which comprises:

A. a continuous phase matrix copolymer comprising, in polymerized formand on a matrix copolymer weight basis, from about 35 to about 89 weightpercent of a monovinylidene aromatic monomer, from about 10 to about 40weight percent of an ethylenically unsaturated nitrile monomer, and fromabout 1 to about 25 weight percent of an N-substituted maleimidemonomer; and

B. dispersed within said matrix copolymer, discrete rubber particleshaving grafted thereon and occluded therein a rigid copolymerconstituent comprising, in polymerized form and on a rigid copolymerconstituent weight basis, from about 25 to about 94 weight percent of amonovinylidene aromatic monomer, from about 5 to about 40 weight percentof an ethylenically unsaturated nitrile monomer, and from about 1 toabout 35 weight percent of a N-substituted maleimide monomer; saidrubber-modified monovinylidene aromatic copolymer composition beingfurther characterized in that it has a swelling index value of at least12 and in that the difference between the N-substituted maleimidemonomer content of the matrix phase copolymer and that of the graftedand occluded rigid copolymer constituent is 9 weight percentage pointsor less.

In another of its aspects, the present invention resides in a processfor preparing an improved maleimide-containing, rubber-modifiedmonovinylidene aromatic copolymer composition. Said process comprisesthe steps of:

A. dissolving a rubbery polymer material in a monomer mixture comprisinga monovinylidene aromatic monomer, an ethylenically unsaturated nitrilemonomer and, optionally, an N-substituted maleimide monomer;

B. partially polymerizing the resulting solution of said rubbery polymermaterial in said monomer mixture;

C. adding an N-substituted maleimide monomer to the partiallypolymerized solution of said rubbery polymer material in said monomermixture when at least 20 weight percent of said monomer mixture has beenconverted from monomer to polymer;

D. continuing to polymerize the partially polymerized reaction mixtureof step (C) to the desired degree of polymerization; and

E. removing any unreacted monomers from the product of step D atelevated temperature and reduced pressure and under conditions such thatthe swelling index of the resulting rubber modified polymer product is12 or greater.

As has been noted above, the resulting rubber-modified copolymercompositions have improved fatigue resistance and impact strengthproperties relative to otherwise comparable compositions which haveswelling indexes of less than 12. Said compositions also exhibitsuperior impact strength properties when compared to compositions whichare essentially the same in all respects except for having maleimidemonomer content differences of greater than 9 weight percentage pointsas between the matrix phase copolymer and the grafted/occluded rigidphase copolymer portions thereof.

"Swelling index" as used herein provides a measure of the degree ofcrosslinking within the dispersed grafted rubber particles of thepolymer composition of interest. It is determined for a givenrubber-modified copolymer by partially dissolving 0.4 grams of thecopolymer in question in 30 cc of a 70:30 volume ratio solvent mixtureof toluene and methyl ethyl ketone and thereafter centrifuging theresulting mixture to remove the undissolved material from it. Theundissolved material (which does not dissolve due to rubber crosslinkingand which will have been caused to swell due to solvent absorption) isthen weighed to determine its initial "wet" or swollen weight; dried ina vacuum to remove all of the solvent therefrom; and thereafter isweighed again to determine the dry weight thereof.

The swelling index is then calculated as the ratio of the wet weight tothe dry weight as determined by the foregoing procedure. That is,##EQU1##

As is noted above, the swelling index value provides a measure of therelative degree of crosslinking which is present in the dispersedrubbery polymers of the composition in question. The more highlycrosslinked the rubber particle is, the less is its capability forswelling and absorbing larger quantities of solvent. Accordingly,relatively lower swelling index values correspond to relatively higherdegrees of crosslinking within the indicated dispersed, grafted rubberparticles.

Conversely, little or no rubber crosslinking in the sample of interesteither results in there being little or no insoluble matter followingdissolution thereof in the mixed toluene/methyl ethyl ketone solventsystem or results in a relatively high swelling index value such as forexample 30 or 40 or more.

For the purposes of the present invention, the percentage of theN-substituted maleimide monomer contained in the grafted and occludedrigid copolymer constituent can be determined as follows:

1. Add 30 ml of a 70/30 volume ratio mixture of methyl ethyl ketone(MEK)/methanol to 1.0 gm of the resin to be analyzed and shake themixture for at least hours.

2. Centrifuge at 19,500 RPM and 5° C. for 2 hours.

3. Pour off the supernatant.

4. Dry the remaining gel phase at 150° C. and ambient pressure for 30minutes, and at 150° C. and 5 mm Hg pressure (absolute) for 60 minutes.##EQU2## 5. Elemental analysis of the gel phase for oxygen content willthen allow determination of the percent of N-substituted maleimidemonomer contained in the graft and occlusions.

The percentage of N-substituted maleimide monomer contained in thematrix phase can be ascertained by first determining the total maleimidemonomer content in the overall resin sample via Fourier TransformInfrared Spectroscopy (FTIR) and then subtracting out the amount whichhas been found via the above-presented procedure to have beenpolymerized into the grafted and occluded copolymer.

DETAILED DESCRIPTION OF THE INVENTION

Monovinylidene aromatic monomers suitable for use in both the matrixphase portion and in the grafted and occluded portion of the subjectcopolymer compositions include styrene, α-methylstyrene, p-vinyltoluene,p-t-butylstyrene, etc. Especially preferred for such usage is styrene.

The amount of monovinylidene aromatic monomer contained in the indicatedmatrix phase copolymer and in the grafted/occluded rigid copolymerportion is typically from about 25 to about 94 weight percent (basedupon the weight of the respective copolymer components) and is morepreferably in the range of from about 35 to about 89 (especially fromabout 50 to about 85) weight percent.

Ethylenically unsaturated nitrile monomers suitable for use hereininclude acrylonitrile and lower alkyl substituted derivatives thereofsuch as methacrylonitrile, ethacrylonitrile, etc. with acrylonitrilebeing especially preferred in most instances. While not beingparticularly critical for the purposes of the present invention, theethylenically unsaturated nitrile monomer content within the matrix andgrafted/occluded copolymer portions of the subject polymer compositionis typically in the range of from about 5 to about 40 weight percent andis preferably in the 10 to about 40 (especially from about 15 to about30) weight percent range.

N-substituted maleimide monomers suitable for use herein include N-alkylmaleimides such as N-methylmaleimide, N-ethylmaleimide,N-propylmaleimide, N-isopropylmaleimide, N-t-butylmaleimide, etc.;N-cycloalkylmaleimides such as N-cyclohexylmaleimide; N-arylmaleimidessuch as N-phenylmaleimide, N-naphthylmaleimide, etc.; and the like.

Typically, the indicated N-substituted maleimide monomer will constitutefrom about 1 to about 35 (preferably from about 1 to about 25 andespecially from about 5 or 10 to about 20) weight percent of therespective matrix copolymer and grafted/occluded copolymer constituents.

As has been mentioned above, one key feature of the present inventionresides in assuring that the N-substituted maleimide monomer content isreasonably well balanced (i.e., numerically differing by 9 weightpercentage points or less) as between the matrix copolymer portion andthe grafted/occluded rigid copolymer portion of the subject polymercomposition.

Preferably, the N-substituted maleimide contents of the indicated matrixand grafted/occluded copolymer components differ from each other by nomore than about 8 (especially no more than about 7) weight percentagepoints. As is noted above and as is seen in the hereinafter presentedworking examples, the impact strength of the indicated polymercompositions (i.e., at a given rubber content and rubber particle size)is substantially decreased when the maleimide content difference betweenthe two phases exceed the aforementioned values.

A second key feature of, or requirement for, the subject polymercompositions is that the swelling index thereof (i.e., reflecting thedegree of crosslinking within the dispersed rubber particles thereof) beat least 12. As is illustrated in the working examples which follow, theimpact strength and fatigue resistance characteristics of suchcompositions are substantially improved at swelling index values in therange indicated as compared to that which is attained at lower swellingindex values. Preferably, the swelling index value of the subjectpolymer composition is in the range of from about 15 to about 20 or 25.

When preparing the compositions hereof by generally known mass, solutionor mass/suspension polymerization techniques, the swelling index of theresulting compositions is largely controlled by the type and amount ofpolymerization initiator employed therein and upon the temperature andresidence time utilized in removing residual volatile materials from thepolymerized reaction mixture. Thus, for the purposes of the presentinvention, it is important to select and control those parameters in afashion such that the aforementioned swelling index requirement is metin the resulting polymer composition.

As is typical of conventional mass, solution or mass/suspensionpolymerized ABS polymer compositions, the rubber content of the subjectimpact modified styrenic copolymer compositions generally falls withinthe range of from about 5 to about 30 (more preferably from about 7 toabout 25 and especially about 9 to about 21) weight percent on a totalcomposition weight basis.

As is also typical of rubber-modified polymers prepared in the foregoingfashion, the dispersed rubber particles of the present polymercompositions, when prepared by mass, solution or mass/suspension graftpolymerization techniques, will generally have a volume average particlesize within the range of from about 0.5 to about 5 (preferably fromabout 0.8 to about 3) micron.

Rubbery polymer materials suitable for use in preparing the subjectimpact-modified monovinylidene aromatic copolymers include homopolymersof conjugated diene monomers such as 1,3-butadiene, isoprene, etc. andcopolymers of such diene monomers with up to about 40 weight percent ofcopolymerizable monoethylenically unsaturated monomers such asmonovinylidene aromatic monomers, ethylenically unsaturated nitrilemonomers, C₁ -C₄ alkyl acrylate or methacrylate monomers, etc. Suchrubbery polymer materials generally have a glass transition temperature(Tg) of less than 0° C. and, most preferably, the Tg thereof is lessthan -20° C. for the present invention's purposes.

Preparation of the subject polymer compositions is suitably conductedgenerally in accordance with known mass, solution or mass/suspensionpolymerization techniques. Thus, the indicated rubbery polymer isinitially dissolved in a monomer mixture containing the desired monomermaterials and optionally containing an organic solvent or diluent andthe resulting rubbery polymer/monomer solution is polymerized(partially) to a point at which phase inversion occurs (i.e., at whichthe dissolved rubbery polymer comes out of the solution and takes theform of discrete rubber particles dispersed within the polymerizingmonomer mixture). Polymerization of the resulting heterogeneous (i.e.,two phase) mixture is then continued until the desired degree ofmonomer-to-polymer conversion has been achieved and the resultingreaction mixture is then devolatilized (typically under elevatedtemperature and reduced pressure conditions) to remove any residualmonomer materials (and any diluent used in the process) and to therebyrecover the desired graft copolymer product.

Although the present invention may be practiced in a batchpolymerization process, it is preferably conducted in a continuousfashion in either a backmixed or a plug flow (non-backmixed) reactor.One such suitable process employs one or more well stirred tubularreactors. Desirably, the polymerization is conducted in a trainconsisting of two and preferably three plug flow, stirred tube reactorsconnected in series. Phase inversion preferably occurs in the firstreactor and the polymerizing mixture is discharged from the firstreactor into the second reactor and subsequent reactors. In theremaining reactors polymerization is continued in the presence ofagitation to the desired final degree of polymerization.

In conducting the indicated polymerization process, it has been observedthat the N-substituted maleimide monomer employed therein exhibits arelatively rapid rate of polymerization. As a result, there tends to bea substantial "composition drift" (i.e., in terms of N-substitutedmaleimide monomer content) as between copolymer formed during the earlystages of the polymerization (e.g., grafted and occluded copolymer andsome of the matrix phase copolymer) and that portion of the continuousmatrix phase copolymer which is formed later in the process.

Thus, for example, it has been found that in those instances wherein allof the N-substituted maleimide monomer component is charged to the graftpolymerization process at or near the very beginning thereof, thedifference in the maleimide monomer content as between the respectivegrafted/occluded copolymer and matrix copolymer portions will typicallybe in the range of 10 weight percentage points or more and can quitecommonly be in the range of 13 to 15 weight percentage points or more.

Accordingly, in order to ensure that the maleimide monomer contents ofthe respective grafted/occluded and matrix copolymer phases aresufficiently balanced for the purposes of this invention, the subjectpolymer compositions are prepared by withholding at least a portion(e.g., from about 20 to 100 weight percent and preferably from about 30to about 75 weight percent) of the N-substituted maleimide 5 monomerfrom the initial monomer charge and by deferring the addition thereofuntil later in the polymerization process.

Typically, it is advantageous to introduce the deferred maleimidemonomer charge or feed stream at a point in the process at which atleast 20 percent (and preferably at least 25 or 30 percent) of theoriginal monomer charge has been converted from monomer to polymer.

Most preferably, the indicated deferred maleimide monomer addition isconducted at a stage in the graft polymerization process which is after(and preferably only shortly after) the rubber phase inversion which hasbeen described above has already occurred.

It is also generally preferred that the indicated deferred maleimidemonomer addition be conducted at stage in the process which is prior to60 (more preferably prior to 50) percent conversion of the originalmonomer charge or feed stream.

Typically, the aforementioned graft polymerization process will beconducted at a temperature in the range of from about 80° to about 180°C. and will employ an effective amount (e.g., from about 50 to about 200ppm) of a conventional polymerization initiator such as, for example,1,1-bis (t-butylperoxy) cyclohexane, dicumylperoxide,t-butylperoxy-2-ethylhexanoate, etc.

Devolatilization of the resulting reaction mixture (i.e., to recover thedesired graft copolymer product) is typically conducted at a temperaturein the range of from about 200° to about 300° C.; at a pressure of about30 millimeters of mercury (absolute) and for a residence time of fromabout 0.2 to about 1.5 hours.

As will undoubtedly be readily-apparent to those skilled in the art, thesubject mass, solution or mass/suspension graft copolymer compositionshereof can be readily compounded with other conventional ingredientssuch as pigments, mold release agents, halogenated fire retardantingredients, fillers, reinforcing materials, blowing agents, otherthermoplastic resin ingredients, etc. as may be desired in a giveninstance.

In one such preferred embodiment hereof, the indicatedmaleimide-modified mass, solution or mass/suspension graft copolymer ismelt compounded with from about 5 to about 40 weight percent (on a totalcomposition weight basis) of an emulsion graft polymerized rubberconcentrate material in order to further enhance the impact strength andtoughness characteristics of the resulting polymer composition. Saidgrafted rubber concentrate typically contains from about 30 to about 70weight percent of dispersed rubber particles (usually having volumeaverage particle sizes in the range of from 0.05 to about 1, preferablyfrom about 0.1 to about 0.6, micron) having grafted thereto a rigidsuperstrate copolymer of a monovinylidene aromatic monomer, anethylenically unsaturated nitrile monomer and, optionally, anN-substituted maleimide monomer.

The present invention is further illustrated and understood by referenceto the following working examples in which all parts and percentages arestated on a weight basis unless otherwise indicated. Within such workingexamples, the various physical properties and characteristics of theresulting polymer compositions were determined as indicated below:

Izod Impact--ASTM D256-87 using injection molded test specimens

Tensile (Tm, Ty, Tr, %E)--ASTM D638-87b using injection molded specimens

Instrumented Dart Drop--ASTM D3763-86 (Injected molded specimens)

Distortion Temperature Under Load (DTUL)--ASTM D648-82

Vicat--ASTM D1525-87

Melt Flow Rate (MFR)--ASTM D 1238-86

Gloss--ASTM D523-89

Molecular Weight (Mw, Mn)--Determined by Gel Permeation Chromatograph(GPC)

Reduced Viscosity--Determined as described at Column 4, lines 40-45 ofU.S. Pat. No. 4,808,661

Fatigue--Tensile fatigue testing using double notched, compressionmolded ASTM tensile bars at 1 cycle per second, peak load of 1,500 psiand minimum:maximum load ratio of 0.1.

EXAMPLE 1

A continuous solution polymerization process is conducted utilizingthree well stirred reactors connected in series. Each reactor is capableof holding 2.8 lbs (1.27 Kg) of reaction mixture and all three of themare operated at full volumetric capacity. A main monomer feed streamcomprising styrene (61 percent) and acrylonitrile (14.9 percent) havingdissolved therein 8.8 percent polybutadiene rubber (Diene 55, availablefrom the Firestone Tire and Rubber Company), 15.4 percent ethylbenzenesolvent, 0.2 percent hindered phenolic antioxidant (Irganox 1076), 138parts per million initiator, 1,1 -bis(t-butyl peroxy)cyclohexane, and100 parts per million of chain transfer agent (n-dodecyl mercaptan) isintroduced to the first reactor at a rate of 0.78 lb/hr. (0.35Kg/hr) Asecond monomer stream containing 40.3 percent acrylonitrile, 26.8percent N-phenylmaleimide (N-PMI) and 32.9 percent ethylbenzene solventis added at three points in the polymerization where the main monomerfeedstream conversion fraction is 0.084, 0.332 and 0.411. The totalamount of this stream (i.e., the so-called "Split N-PMI Addition" feedstream) is 0.079 lb/hr (0.036 Kg/hr) and is added in equal amounts ateach of the three points of addition. An additional 900 parts permillion of chain transfer agent (on a main monomer feedstream basis) isadded at the point in the reactor train where monomer conversion is0.332. The first reactor is maintained at an average temperature of 104°C. with stirring. Phase inversion occurs in the first reactor (at apoint where monomer conversion is approximately 17 percent) and theeffluent is charged to thesecond reactor in the series. The temperatureof the second reactor is maintained at 125° C. The product from thesecond reactor is charged to the third reactor which is maintained at atemperature of 145° C. The final product is devolatilized to removeunreacted monomer and solvent and is then pelletized. The properties ofthe resulting polymer are shown in Table 1.

Comparative Example 1-A

The polymerization process and reaction conditions of Example 1 aresubstantially repeated with the exception that all the N-phenylmaleimideis added in the monomer feed stream before it enters the firstpolymerizer. The properties of the resulting polymer are shown in Table1.

Comparative Example 1-B

The graft copolymer of Example 1 is fed to a twin screw extruderoperating at an average temperature of 200° C. along with 0.1 percent of2,5-dimethyl-2,5-di-(t-butyl peroxy) hexane (Lupersol 101) in order tofurther crosslink the rubber component thereof and to thereby reduce theswelling index of the resulting composition. The properties of theresulting polymer are shown in Table 1.

Comparative Example 1-C

The graft copolymer of Comparative Example 1-A is also fed to a twinscrew extruder operating at an average temperature of 200° C. along with0.1 percent of 2,5-dimethyl-2,5-di-(t-butyl peroxy) hexane (Lupersol101) to reduce the swelling index of that graft copolymer. Theproperties of the resulting polymer are shown in Table 1.

EXAMPLE 2

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 53.6 percent styrene, 16.8 percentacrylonitrile, 0.9 percent N-phenylmaleimide, 12.0 percent of astyrene-butadiene diblock rubber (Stereon 730A from Firestone Tire andRubber Company) and 16.4 percent ethylbenzene solvent. The secondmonomer stream is identical in composition to Example 1 and is added atthe same points of conversion as in Example 1 but at a total rate of0.095 lb/hr. The properties of the resulting polymer are shown in Table1.

Comparative Example 2

The graft copolymer of Example 2 was fed to a twin screw extruderoperatingat an average temperature of 200° C. along with 0.1 percent of2,5-dimethyl-2,5-di-(t-butyl peroxy) hexane (Lupersol 101) to crosslinkthe rubber particles thereof and to thus reduce the swelling index ofthe resulting polymer composition. The properties of the resultingpolymer areshown in Table 1.

                                      TABLE I                                     __________________________________________________________________________                      Comparative                                                                          Comparative                                                                          Comparative  Comparative                      Experiment Number                                                                         Example 1                                                                           Example 1-A                                                                          Example 1-B                                                                          Example 1-C                                                                          Example 2                                                                           Example 2                        __________________________________________________________________________    Split N-PMI Addition*                                                                     Yes   No     Yes    No     Yes   Yes                              Percent of total N-PMI                                                                    66.7  None   66.7   None    51    51                              added after phase                                                             inversion                                                                     Swelling Index                                                                            16.4   15    10.1   9.55   18.2  10.1                             Mw          164,100                                                                             170,900                                                                              145,200                                                                              146,300                                                                              147,300                                                                             124,600                          Mn           64,080                                                                              63,670                                                                               59,500                                                                               57,260                                                                               64,140                                                                              56,140                          Reduced Viscosity                                                                          0.79 0.75    0.70  0.71   na    na                               (cm3/g)                                                                       MFR (g/10 min)                                                                              3   2.3     4.1   3.7    2.89   4.1                             Tm (psi)    305,000                                                                             355,000                                                                              305,000                                                                              312,000                                                                              288,000                                                                             263,000                          Ty (psi)    5907  6236   5937   6282   6484  6333                             Tr (psi)    4936  5028   5287   5407   5160  5374                             % Elongation (%)                                                                          56.9   17    75.5   25.2   25.1  49.6                             Vicat/DTUL (°F.)                                                                   236/180                                                                             237/190                                                                              236/182                                                                              241/186                                                                              235/172                                                                             232/174                          Izod (ft lb/in)                                                                            2.76 1.82    1.89  1.4     3.76  2.02                            Dart Impact RT (total)                                                                      197  106    133    92     333   210                             (in-lb)                                                                       Dart Impact -20° F.                                                                 112   76     54     43     87    77                              (total) (in-lb)                                                               Gloss        65    70     51     53    66.5  51.2                             Fatigue (cycles to                                                                        6140  na     4440   na     9100  5600                             fail)                                                                         __________________________________________________________________________    *Connotes postphase inversion addition of at least a portion of the NPMI       monomer.                                                                 

As can be seen from the results in Table 1, the addition of at least aportion of the N-phenylmaleimide (N-PMI) monomer component at a laterstage in the polymerization process (i.e., after the point of rubberphaseinversion as in the case of Examples 1 and 2) provides notablyimproved impact strength results relative to those which are obtained byadding allof N-PMI monomer in the beginning stages of the polymerizationprocess (i.e., pre-phase inversion).

As can also be seen (i.e., by comparing Example 1 with ComparativeExample 1B and Example 2 with Comparative Example 2), the graftcopolymer compositions of the present invention (i.e., having a swellingindex of atleast 12) have notably improved fatigue resistance and impactstrength relative to comparable compositions that have swelling indexesof less than 12.

EXAMPLE 3

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 59.3 percent styrene, 14.2 percentacrylonitrile, 6.3 percent N-phenylmaleimide, 6.0 percent Diene 55polybutadiene rubber and 14.3 percent ethylbenzene solvent. The secondmonomer stream is identical in composition to that of Example 1 and isadded in equal amounts at conversion points of 0.3 and 0.4 at a totalrateof 0.13 lb/hr (0.059 Kg/hr). The resulting graft copolymer iscompounded with 19% of a grafted rubber concentrate for additionalimpact resistance and 1% of a bis-stearamide wax. The properties of theresulting polymer are shown in Table 2. The grafted rubber concentrateemployed in this example is one in which styrene and acrylonitrile (in a70:30 weight ratio) have been emulsion graft polymerized to a 93:7weight ratio butadiene/styrene copolymer latex. Such grafted rubberconcentrate has a rubber content of 52 weight percent, a volume averagedparticle size of 0.15 micron and a grafted copolymer to rubber ratio of0.3.

Comparative Example 3-A

Example 3 is substantially repeated with the exception that the mainfeed, containing styrene (50.7 percent) and acrylonitrile (17.9 percent)having dissolved therein 5.1 percent polybutadiene Diene 55 rubber, 17.0percent ethylbenzene solvent, has all of the N-phenylmaleimide monomer(9.3 percent)added to it before it enters the first polymerizer. Thisgraft copolymer is also compounded with 19 percent of the grafted rubberconcentrate of Example 3 above for additional impact resistance and 1percent of a bis-stearamide wax. The properties of the resulting polymerare shown in Table 2.

Comparative Example 3-B

The polymerization process of Example 1 is substantially repeated withthe exception that the main monomer feed stream contains styrene (65percent),acrylonitrile (11.6 percent) and N-phenylmaleimide (4.4percent) having dissolved therein 6.6 percent Diene 55 polybutadienerubber and 12.5 percent ethylbenzene solvent. The second monomer streamis identical in composition to Example 1 and is added at the same pointsof conversion butat a total rate of 0.2 lb/hr (0.091 Kg/hr). Theresulting graft copolymer is melt compounded with 19 percent of thegrafted rubber concentrate of Example 3 above for additional impactresistance and 1 percent of a bis-stearamide wax. The properties of theresulting polymer are shown in Table 2.

EXAMPLE 4

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 62.1 percent styrene, 12.9 percentacrylonitrile and 5.4 percent N-phenylmaleimide, 6.2 percent Diene 55polybutadiene rubber and 13.4 percent ethylbenzene solvent. The secondmonomer stream is identical in composition to Example 1 and is added inequal amounts at main monomer feed conversion points of 0.3 and 0.4 at atotal rate of 0.17 lb/hr (0.077 Kg/hr). The resulting graft copolymer iscompounded with 19 percent of grafted rubber concentrate of Example 3#or additional impact resistance and 1 percent of a bis-stearamide wax.The properties of the resulting polymer are shown in Table 2.

EXAMPLE 5

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 6.6 percent styrene, 15.3 percentacrylonitrile, 7.3 percent N-phenylmaleimide, 5.7 percent Diene 55polybutadiene rubber and 15.2 percent ethylbenzene solvent. The secondmonomer stream is identical in composition to Example 1 and is added inequal amounts at main feed monomer conversion points of 0.3 and 0.4 at atotal rate of 0.095 lb/hr (0.043 Kg/hr). The resulting graft copolymeris compounded with 19 percent of the grafted rubber concentrate ofExample 3 for additional impact resistance and 1 percent of abis-stearamide wax. The properties of the resulting polymer are shown inTable 2.

                                      TABLE 2                                     __________________________________________________________________________                          Comparative                                                                          Comparative                                      Experiment Number                                                                             Example 3                                                                           Example 3-A                                                                          Example 3-B                                                                          Example 4                                                                           Example 5                           __________________________________________________________________________    Split N-PMI Addition*                                                                         Yes   No     Yes    Yes   Yes                                 Percent Rubber   7.9   7.4    7.1    7.6   7.2                                Percent of total N-PMI added                                                                  41.5   0     40.8   51.9  31                                  after phase inversion                                                         Percent of total N-PMI in                                                                     19.9  23.4   24.9   18.4  19.7                                grafted/occluded copolymer                                                    phase                                                                         Percent of total N-PMI in                                                                     12.3   9.8   13.4   13.2  13.3                                matrix phase                                                                  Difference in N-PMI content                                                                    7.6  13.6   11.5   >5.2   6.4                                between grafted/occluded                                                      copolymer and matrix                                                          copolymer                                                                     Izod Impact Strength (ft                                                                       5.29  2.33   3.14   4.56  4.80                               lb/in)                                                                        Room Temperature                                                                              199   172    na     225   na                                  Dart Impact Strength (in-lb)                                                  Low Temperature (-20°F.) Dart                                                           50    19    na      69   na                                  Impact Strength (in-lb)                                                       Room Temperature Dart                                                                         12.1  10.8   na     13.9  na                                  Impact Strength per Percent                                                   Rubber (in-lb/%)                                                              __________________________________________________________________________    *Connotes that at least a portion of the NPMI is added following rubber        phase inversion.                                                         

As can be seen from the data in Table 2, compositions which have a N-PMIcontent difference of less than 9 percentage points as between thegrafted/occluded copolymer portion and the matrix phase portion thereofexhibit substantially enhanced impact strength properties.

Comparative Example 4

Example 1 is substantially repeated with the exception of the mainmonomer feed stream contains 60.1 percent styrene, 15.0 percentacrylonitrile, 9.4percent Diene 55 polybutadiene rubber and 15.5 percentethylbenzene solvent. The second monomer stream is identical incomposition to Example 1 and is added at a main monomer feed conversionpoint of 0.08 at a rate of 0.15 lb/hr (0.068 Kg/hr). The properties ofthe resulting polymer are shown in Table 3.

EXAMPLE 6

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 60.3 percent styrene, 14.9 percentacrylonitrile, 9.5 percent Diene 55 polybutadiene rubber and 15.4percent ethylbenzene solvent. The second monomer stream is identical incomposition to Example 1 is added at conversion points of 0.08, 0.3 and0.4. The total amount of this stream is 0.18 lb/hr (0.082 Kg/hr) and isadded in equal amounts at the three conversion point locations. Theproperties of the resulting polymer are shown in Table 3.

EXAMPLE 7

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 60.3 percent styrene, 14.9 percentacrylonitrile, 9.5 percent Diene 55 polybutadiene rubber and 15.4percent ethylbenzene solvent. The second monomer stream is identical incomposition to Example 1 and is added at conversion points of 0.08, 0.3and 0.4 in amounts of 0.12, 0.03 and 0.03 lb/hr (0.055, 0.014 and 0.014Kg/hr), respectively. The properties of the resulting polymer are showninTable 3.

                  TABLE 3                                                         ______________________________________                                                       Compara-                                                                      tive                                                           Experiment Number                                                                            Example 4 Example 6 Example 7                                  ______________________________________                                        Split N-PMI Addition*                                                                        No        Yes       Yes                                        Percent Rubber 11.2      11.5      11.5                                       Percent of total N-PMI                                                                        0        66.6      33.3                                       added after phase                                                             inversion                                                                     Percent of total N-PMI                                                                       20.3      13.2      15.3                                       in grafted/occluded                                                           copolymer phase                                                               Percent of total N-PMI                                                                        5.5       8.1       7.2                                       in matrix phase                                                               Difference in N-PMI                                                                          14.8       5.1       8.1                                       content between                                                               grafted/occluded                                                              copolymer and matrix                                                          copolymer                                                                     Izod Impact Strength                                                                          2.10      2.30      2.42                                      (ft lb/in)                                                                    Room Temperature                                                                             102       183       146                                        Dart Impact Strength                                                          (in-lb)                                                                       Low Temperature                                                                               99        70        75                                        -20° F.) Dart Impact                                                   Strength (in-lb)                                                              Room Temperature Dart                                                                         9.1      15.9      12.7                                       Impact Strength per                                                           Percent Rubber                                                                ______________________________________                                        *Connotes that at least a portion of the NPMI monomer is added following       rubber phase inversion.                                                  

As can be seen from the results in Table 3, substantially better roomtemperature Dart impact strength results are obtained when at least aportion of the maleimide monomer is added after rubber phase inversionthereby causing the maleimide monomer contents of the grafted/occludedcopolymer and the matrix phase copolymer to be within 9 percentagepoints of each other.

EXAMPLE 8

Example 1 is substantially repeated with the exception that astyrene-butadiene diblock rubber (Stereon 730A) is used in place of thebutadiene homopolymer rubber of Example 1. The properties of theresultingpolymer are shown in Table 4.

Comparative Example 8

Comparative 1-A is also substantially repeated with the exception ofusing the Stereon 730A block copolymer rubber in place of Diene 55butadiene homopolymer rubber. The properties of the resulting PO polymerare shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                                      Comparative                                     Experiment Number  Example 8  Example 8                                       ______________________________________                                        Split N-PMI Addition*                                                                            Yes        No                                              Percent Rubber     11.8       11.9                                            Percent of total N-PMI added                                                                     66.6        0                                              after phase inversion                                                         Percent of N-PMI in                                                                              10.7       14.5                                            grafted/occluded copolymer                                                    phase                                                                         Percent of N-PMI in matrix                                                                        3.9        4.7                                            phase                                                                         Difference in N-PMI content                                                                       6.8        9.8                                            between grafted/occluded                                                      copolymer and matrix                                                          copolymer                                                                     Izod Impact Strength (ft                                                                          2.76       1.82                                           lb/in)                                                                        Room Temperature   197        106                                             Dart Impact Strength (in-lb)                                                  Low Temperature (-20° F.) Dart                                                            112         76                                             Impact Strength(in-lb)                                                        Room Temperature Dart Impact                                                                     16.7        8.9                                            Strength per Percent Rubber                                                   Swelling Index      15         16                                             ______________________________________                                        *Connotes postphase inversion addition of at least a portion of the NPMI       monomer.                                                                 

EXAMPLE 9

Example 1 is substantially repeated with the exception that the mainmonomer feed stream contains 57.8 percent styrene, 14.0 percentacrylonitrile, 12.7 percent Stereon 730A styrene-butadiene diblockrubber and 15.5 percent ethylbenzene solvent and is added at a rate of0.89 lb/hr(0.405 Kg/hr). The second monomer stream is identical incomposition to that of Example 1 and is added at main feed conversionpoints of 0.09, 0.3and 0.55. The total amount of this stream is 0.18lb/hr (0.082 Kg/hr) and is added in equal amounts at the threeconversion point locations. The properties of the resulting polymer areshown in Table 5.

EXAMPLE 10

A modification to the process described in Example 1 results in thefirst and second stirred reactors being continuously recirculated (or"backmixed") at a rate of 21:1 (material being recirculated:fresh feed).The recirculation stream has monomer to polymer conversion of 41 percentand the main monomer feed stream to the recirculated reactors contains48.3 percent styrene, 18.4 percent acrylonitrile, 10.6 percent Stereon730A rubber, and 18.0 percent ethylbenzene, and 190 parts per millioninitiator, 1,1-bis(t-butyl peroxy)cyclohexane. The temperature of therecirculated reactors is maintained at 103° C. with stirring.Phaseinversion of the freshly added feed is instantaneous. The thirdreactor, which is not recirculated, is maintained at a temperature of130° C. The properties of the resulting polymer are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Experiment Number  Example 9  Example 10                                      ______________________________________                                        Percent Rubber     15.7       15.6                                            Percent of total N-PMI                                                                           66.7       100                                             added after phase                                                             inversion                                                                     Percent of Total N-PMI in                                                                        11.4       12.1                                            grafted/occluded copolymer                                                    phase                                                                         Percent of total N-PMI in                                                                         7.7        7.5                                            matrix phase                                                                  Difference in N-PMI                                                                               3.7        4.6                                            content between                                                               grafted/occluded copolymer                                                    and matrix copolymer                                                          Izod Impact Strength (ft                                                                          3.2        1.9                                            lb/in)                                                                        Room Temperature   330        230                                             Dart Drop Impact Strength                                                     (in-lb)                                                                       Low Temperature (-20° F.)                                                                 116         33                                             Dart Impact Strength (in-lb)                                                  Room Temperature Dart Impact                                                                      21        14.7                                            Strength per percent Rubber                                                   (in-lb per %)                                                                 Swelling Index     19.5       12.5                                            ______________________________________                                    

As can be seen from the results in Table 5, the resin prepared using therecirculation or backmixing technique of Example 10 resulted in a resinhaving a difference of only 4.6 percent as between the N-PMI content ofthe grafted/occluded copolymer and the matrix phase. It can also beseen, however, that utilization of the split feed/post phase inversionN-PMI addition in the context of a plug flow (non-backmixed) graftpolymerization process provided relatively tougher final product havingnotably better Izod impact strength and Dart impact strengthcharacteristics.

While the present invention has been described and illustrated withreference to certain specific and preferred embodiments thereof, such isnot to be interpreted as in any way restricting or limiting the scope ofthe instantly claimed invention.

What is claimed is:
 1. A rubber-modified monovinylidene aromaticcopolymer composition which comprises:A. a continuous phase matrixcopolymer comprising, in polymerized form and on a matrix copolymerweight basis, from about 35 to about 89 weight percent of amonovinylidene aromatic monomer, from about 10 to about 40 weightpercent of an ethylenically unsaturated nitrile monomer, and from about1 to about 25 weight percent of an N-substituted maleimide monomer; andB. dispersed within said matrix copolymer, discrete particles of aconjugated diene homopolymer or copolymer rubber having grafted thereonand occluded therein a rigid copolymer constituent comprising, inpolymerized form and on a rigid copolymer constituent weight basis, fromabout 25 to about 94 weight percent of a monovinylidene aromaticmonomer, from about 5 to about 40 weight percent of an ethylenicallyunsaturated nitrile monomer, and from about 1 to about 35 weight percentof a N-substituted maleimide monomer; said rubber-modifiedmonovinylidene aromatic copolymer composition being furthercharacterized in that it has a swelling index value of from about 15 toabout 25 and in that the difference between the N-substituted maleimidemonomer content of the matrix phase copolymer and that of the graftedand occluded rigid copolymer constituent is 9 weight percentage pointsor less.
 2. The aromatic copolymer composition of claim 1 wherein theN-substituted maleimide monomer of both matrix copolymer and the graftedand occluded rigid copolymer is an N-aryl maleimide.
 3. The aromaticcopolymer composition of claim 2 wherein the N-aryl maleimide isN-phenyl maleimide.
 4. The aromatic copolymer composition of claim 3wherein, in both the matrix copolymer and the grafted and occludedcopolymer, the monovinylidene aromatic monomer is styrene and theethylenically unsaturated nitrile is acrylonitrile.
 5. The aromaticcopolymer composition of claim 1 wherein the difference between theN-substituted maleimide content of the matrix phase and theN-substituted maleimide content of the grafted and occluded rigidcopolymer is 8 weight percentage points or less.
 6. The aromaticcopolymer composition of claim 1 wherein the swelling index thereof isfrom about 15 to about
 20. 7. The aromatic copolymer composition ofclaim 1 wherein the rubber content thereof is from about 5 to about 30percent based upon the total weight of said composition.
 8. The aromaticcopolymer composition of claim 1 wherein said composition has beenprepared by a mass, solution or mass/suspension polymerization process.9. The aromatic copolymer composition of claim 8 wherein saidcomposition further comprises, on a total composition weight basis, fromabout 5 to about 40 weight percent of an emulsion graft polymerizedrubber concentrate which comprises, on a rubber concentrate weightbasis, from about 30 to about 70 weight percent of dispersed rubberparticles having grafted thereto a rigid superstrate polymer comprising,in polymerized form, a monovinylidene aromatic monomer, an ethylenicallyunsaturated nitrile monomer and, optionally, an N-substituted maleimidemonomer.